The Worldwide Interoperability for Microwave Access (WiMAX) standards are a family of communication standards defining high-bit rate connections by radio channel for point-to-multipoint architectures. The WiMAX standards notably include the standards of the IEEE 802.16 family. The standards of the IEEE 802.11 family, also known as “WiFi standards”, are the international standard describing the characteristics of a wireless local area network (WLAN) capable of communicating at high bit rates over a radius of several meters.
In Digital Video Broadcasting (DVB), the mismatches of the two channels in phase quadrature (channels I and Q according to the designation used by those skilled in the art) of the transmission subsystem (defects also designated by those skilled in the art as “IQ mismatch”) and the transposition signal leaks from a frequency transposition stage (also known by those skilled in the art by the acronym “LO leakage”) are defects that are known in the transmission subsystems of radio frequency devices. These subsystems have architecture of the direct-conversion type, also called Zero Intermediate Frequency (ZIF), or an architecture of the superheterodyne type, i.e. with a low or high non-zero intermediate frequency.
Generally, the mismatches of the two channels in quadrature are of two different types, namely a phase mismatch and a gain mismatch. The gain mismatch originates in all the stages having a gain extending from the digital-to-analog converter stage to the first frequency transposition stage. This is because, in this area, the I and Q channels are amplified separately. The phase mismatch originates in the generation of the frequency transposition signals used by the mixers situated on the I and Q channels. These signals are not, in practice, strictly mutually orthogonal in phase, i.e. not strictly in phase quadrature.
Regarding the defect due to the transposition signal leaking, this originates in the transposition stage and is characterized by an unwanted frequency component situated in the center of the radio frequency band. The compensation and the calibration of the mismatches and of the transposition signal leaks have hitherto been the subject of several different approaches.
A first approach comprises eliminating the quadrature error in the design of the circuit for generating frequency transposition signals in quadrature. However, such approaches are complicated and can lead to an inadequate rejection of the image signal for high-order modulations, such as the 64 QAM and 256 QAM modulations. Factory calibration is also an approach for correcting the imperfections mentioned hereinabove. The measured mismatches are either compensated by analog means, for example, by resistors and variable capacitors, or through the intermediary of a digital pre-distortion, the control value of which is stored in a non-volatile memory.
However, a factory calibration is costly and does not make it possible to take into account the variations of these mismatches associated with temperature and power supply voltage variations. Thus, other approaches recommend an on-chip calibration. Such approaches are generally based on a quadratic measurement of the power envelope of the signal leading to an iterative estimation of the mismatches. However, such an iterative search is lengthy, which can prove incompatible with the particular requirements of certain standards, such as the WiMAX standards. Furthermore, it is difficult to obtain a good quality envelope detector. Other approaches recommend statistical calculations on the whole signal sent in order to determine the mismatches and the local oscillators leaks. However, these approaches are also costly in terms of time.